1. Field of the Invention
The present invention relates to a liquid crystal display device having a light-tight structure and a liquid crystal projector using the same.
2. Description of the Related Art
Liquid crystal displays include direct-view type displays and projection displays (referred to as liquid crystal projectors in the following description) for throwing an image onto a screen for display. The liquid crystal projectors include color liquid crystal projectors. The color liquid crystal projectors allow light emitted from a single white light source to be separated into three primary colors for color display by a color separating means. Each primary color light is then introduced to pixels (liquid crystal cells) of a liquid crystal display device corresponding to each color. The light is modulated therein in accordance with an image to be reproduced to be projected onto a screen for color image display. Such liquid crystal projectors fall into two broad categories: single-panel and triple-panel types. The single-panel type comprises a single liquid crystal display device (liquid crystal panel) having a color separating means for red, green and blue such as a dichroic mirror. The triple-panel type comprises a total of three monochrome liquid crystal panels each placed in an optical path of the respective colors, that is, red, green and blue.
The liquid crystal projectors as described so far generally have a metal halide lamp emitting intense light, for example, for a light source. The light emitted from the metal halide lamp is reflected forward by a spheroidal reflecting mirror. In front of the light source a heat-ray cut-off filter, an incident polarizing plate and a condenser lens are placed in this order. In front of the condenser lens a liquid crystal display device, an outgoing polarizing plate, a projection lens and a projection screen are placed. The heat-ray cut-off filter removes unwanted infrared radiation from the intense light emitted from the light source (the metal halide lamp). The light then passes the incident polarizing plate to be gathered by the condenser lens to enter the liquid crystal display device. The light is modulated therein in accordance with an image to be reproduced. The light modulated in the liquid crystal display device passes through the outgoing polarizing plate to be projected in magnification by the projection lens. An image is thereby projected onto the projection screen in front.
However, the liquid crystal projectors as described so far have problems resulting from heat produced by light from the light source. That is, a liquid crystal display device used in such liquid crystal projectors is on the order of 2 mm in thickness and relatively thin. As a result, when there are inconsistencies in intensity of light from the light source, the light may locally concentrate to partially heat the liquid crystal display device. That is, a `hot spot` may be produced. The hot spot has a transmission factor different from surrounding regions. Therefore, the quality of a projected image in magnification will be reduced to a great extent. Another problem is that a temperature of the liquid crystal display device rises due to radiation heat from the light source and intensive light strikes drive elements where thin film transistors (TFT) and so on are formed. Both heat and light thus reduce liquid crystal properties and thereby the display function does not work properly.
Therefore, liquid crystal projectors of, the related art incorporate a mechanism for cooling a liquid crystal display device. For the cooling mechanism an air-cooling type for cooling by means of blowing air and a liquid cooling type for cooling with liquid are adopted.
However, air-cooling by blowing air also blows dust in a liquid crystal display apparatus. If dust settles on a liquid crystal display device and a lens, it will be projected onto a screen when projected by a liquid crystal projector. Furthermore, if air is increased for satisfactory cooling, noise also will increase due to high-speed rotation of a fan. The size of the fan may be further enlarged which will result in upsizing of the device. On the other hand, an example of the liquid-cooling type mechanism is disclosed in Japanese Patent Publication 6-58474 (1994). However, the liquid-cooling type requires liquid to be filled for a heat exchanging medium. This type therefore incorporates problems of reliability relating to release of pressure because of a rise in the temperature, air bubbles, mixing of foreign substances, leakage of cooling liquid and so on. For a water-cooling type in particular, the formation of rust and the like on metal parts is a problem, too. The liquid-cooling type requires upsizing of the cooling mechanism itself since a large amount of liquid is required for cooling. Another cooling system is a solid-cooling type equipped with an electronic cooling device such as a Peltier element. This type, however, increases costs of the overall liquid crystal projector while the cooling effect thereof is not satisfactory.